A transducer is a device that converts one type of energy into another type of energy for the purpose of measurement or information transfer. A sensor transducer is a type of transducer that detects (senses) a signal or physical condition and converts it to a signal that can be read and analyzed by humans. Examples of devices that use sensor transducers include mass airflow sensors, speed sensors, position sensors, pressure sensors, relative humidity sensors, and the like.
In certain situations a combination sensor or “combi-sensor” is used to measure multiple signals or physical conditions using a single sensor device. Combi-sensors often incorporate one or more sensor transducers that measure flow (e.g., air-flow, water-flow, etc.). Flow sensors can have highly non-linear outputs because their output is dependent upon many factors such as the temperature coefficients of the resistance of the sensing element, thermal transfer characteristics of the media being measured and the media of the transducer, and the mechanical dimensions of the flow path.
As is well known, the output of a sensor transducer, referred to herein as a “raw signal”, must be conditioned so that it can be properly used by an end-user. Signal conditioning circuits and conditioning techniques (also referred to as “signal compensation” or “signal correction”) condition raw signals from sensor transducers, regardless of the quantity being measured by the sensor transducer or the sensor transducer technologies. When a combi-sensor includes a flow-sensor, the high level of non-linearity of the output requires that the conditioning scheme also be highly non-linear. Other factors, such as the ambient temperature around the sensors and the sensitivity of the various sensing technologies can also affect the linearity and stability of the signal output from a sensor transducer, further adding to the need to provide non-linear conditioning capability for the output signal.
Application Specific Integrated Circuits (ASICs) have been developed for conditioning sensor transducer signals, and these ASICs offer a wide variety of programming options that can be specifically tailored to match the characteristics of the particular sensor technology. Because there are so many different types of sensors on the market (pressure, airflow, speed, position, etc.), it is practically impossible to design an affordable ASIC capable of conditioning the raw signals output from every type of transducer. However, in most cases raw signals need to be conditioned for similar characteristics (sensitivity, offset, temperature induced sensitivity changes, temperature induced offset changes and non-linear characteristics) and thus generic conditioning circuits with the ability to “coarsely” condition raw signals for these basic characteristics have been developed. Coarse conditioning as used herein refers to conditioning of a signal using lower order polynomial expressions, e.g., 2nd order polynomial expressions or lower. Typical conditions for which coarse conditioning would be appropriate include compensating a signal for sensitivity changes due to temperature or signal offset changes due to temperature.
Currently, sensor manufacturers are using two methods to condition raw signals output from the sensor transducers of a combi-sensor and deliver them to the user, each of which is advantageous in its own way. In a first method, a signal conditioning ASIC includes a conditioning circuit capable of coarsely conditioning the raw signal and delivers this coarsely-conditioned signal to the end-user. Since the basic level of conditioning is provided by the ASIC, the end-user need not provide or use its own processors to perform conditioning, thereby freeing them up for other tasks. A drawback, as described above, is that the robustness of the conditioning is limited in favor of having a signal conditioning chip that can be used in a wide variety of applications. This technique is adequate for fairly linear outputs but is inadequate for the non-linear outputs of flow sensors and combi-sensors employing flow sensors.
A second method is to provide the end-user with downloadable compensation coefficients that are applied to conditioning equations processed by the processor(s) of the end-user device receiving a raw signal from a sensor. In practice, memory such as a TEDS (Transducer Electronic Data Sheet) IC stores downloadable coefficients that can be used in applications such as signal conditioning applications. A sensor transducer outputs a raw signal to the end-user device, and the optimal coefficients that have been downloaded from the memory are used by a processor in the end-user's system to apply to equations that perform the desired conditioning. Using downloadable coefficients from a memory location gives an end-user the flexibility to, when needed, use higher order (e.g., 3rd order polynomial expressions or greater) exponential functions to condition the raw transducer signals, instead of having to use the more generic conditioning coefficients provided by the signal-conditioning ASIC described above. However, since the end-user performs the conditioning process on the raw signal coming directly from the sensor transducer, the end-user must tie up its processors for conditioning purposes.
It would be desirable to have a flow sensor and/or combi-sensor that incorporates an integrated circuit that can be customized to the needs of a particular end-user and provide to the end-user both a coarsely-conditioned signal to the end-user and downloadable coefficients needed to provide high level conditioning when needed.